Current levels of greenhouse gases in the atmosphere are higher than at any time in the past 650,000 years, say researchers who have finished cataloguing air bubbles trapped for millennia inside Antarctic ice. The record, which extends back over the past eight ice ages, shows that today's concentrations of carbon dioxide and methane far outstrip those in the past.

Atmospheric carbon dioxide levels have risen 200 times faster over the past 50 years than at any other time during this period, says Thomas Stocker of the University of Bern, Switzerland, who led the analysis.

The researchers studied air bubbles preserved in ice drilled from the Antarctic ice sheet as part of the European Project for Ice Coring in Antarctica (EPICA). The ice core represents a logbook of the state of the world's climate (see ' Frozen time') and goes back 210,000 years further than previous records.

After searching ice spanning the period of 390,000-650,000 years before present, Stocker's team has discovered that carbon dioxide levels in the atmosphere did not exceed 290 parts per million during that time. Today, that figure is around 375 parts per million.

The situation is similar for methane: during this period, levels hovered around 600 parts per billion. Today's atmospheric methane concentration is well over 1,700. Stocker and his colleagues report the results in Science1,2.

Unprecedented push

The burning of fossil fuels in the industrial era has pushed greenhouse-gas levels far beyond their natural fluctuations, says Stocker. "This is really something unprecedented," he says. Humans, by releasing fossil fuels from their imprisonment underground, are now adding greenhouse gases to the atmosphere on top of those released as part of natural climate cycles.

The news comes as world leaders plan to attend a United Nations climate change conference in Montreal, Canada, which begins on 28 November. Delegates will discuss current efforts to reduce greenhouse emissions, and what plans should follow on from the initial phase of the Kyoto Protocol, which ends in 2012.

The past four ice ages and their intervening warm periods are thought not to have been typical. Glacial cycles before this had longer, cooler intervening periods than more recent ones. Researchers are unsure why this is, although they hope the ice cores may hold some clues.

Unnatural changes

The newly analysed ice does show that although the climate is in constant flux, it is capable of producing extended warm phases even when carbon dioxide levels are stable, says Stocker. Two places in the record, for example, are marked by periods of almost 30,000 years when temperature hardly changed at all. And the beginning of these 'interglacial' phases was not linked to rises in carbon dioxide.

That's not to say that current rises in temperature are due to natural shifts, as some climate-change sceptics have claimed. "The CO2 emitted now is not part of the natural cycle," Stocker points out.

"In the palaeorecord there's no human activity driving the change," says Chris Jones, of the Hadley Centre for Climate Prediction and Research in Exeter, UK. The current challenge facing climate modellers is to work out the one-way effect of the huge spike in greenhouse gases now being pumped into our skies by human activities.